1. Field of the Invention
The present invention relates to an electronic logic system of the type including a plurality of logic or switching circuits wherein at least some of the logic or switching circuits include devices formed from amorphous semiconductor alloy materials. The system of the present invention provides high speed operation of the logic or switching circuits at rates not heretofore obtained with amorphous semiconductor devices.
2. Description of the Prior Art
Silicon is the basis of the huge crystalline semiconductor industry and is the material which is utilized in substantially all the commercial integrated circuits now produced. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type.
The semiconductor fabrication processes for making p-n junction crystals involve extremely complex, time consuming and expensive procedures as well as high processing temperatures. Thus, these crystalline materials used in transistors and other current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, where p-n junctions are required by doping such single crystals with extremely small and critical amounts of dopants. These crystal growing processes produce relatively small crystal wafers upon which the integrated circuits are formed.
In conventional integrated circuit technology the small area crystal wafer limits the overall size of the integrated circuit which can be formed thereon. In applications requiring large scale areas, such as in the display technology, the crystal wafers cannot be manufactured with as large areas as required or desired. The devices are formed, at least in part, by diffusing p or n-type dopants into the substrate. Further, each device is formed between isolation channels which are diffused into the substrate. Packing density (the number of devices per unit area of wafer surface) is also limited on the silicon wafers, because of limitations in leakage current in each device and the power necessary to operate the devices, each of which generate heat which is undesirable. The silicon wafers do not readily dissipate heat. Also, the leakage current adversely affects the battery or power cell lifetime in portable applications.
In MOS type circuitry the switching speed is related directly to the gate length with the smallest length having the highest speed. The diffusion processes, photolithography and other crystalline manufacturing processes limit how short the gate length can be made.
Further, the packing density is extremely important because the cell size is exponentially related to the cost of each device.
In summary, crystal silicon transistor and integrated circuits have parameters which are not variable as desired, require large amounts of material, high processing temperatures, are only producible on relatively small area wafers and are expensive and time consuming to produce. Devices based upon amorphous silicon alloys can eliminate these crystal silicon disadvantages. Amorphous silicon alloys are easier to manufacture than crystalline silicon and can be manufactured at lower temperatures and in larger areas.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be doped to form p-type and n-type materials to form p-n junction transistors and devices superior in cost and/or operation to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films resulted in such films not being successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands making them unsuitable for making p-n junctions for transistors and other current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C. ). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce extrinsic amorphous material a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited included supposedly substitutional phosphorus or boron dopant and was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter deposition of amorphous silicon films in the atmosphere of a mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as a compensating agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above described p and n dopant materials also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process.
Various methods of fabrication and construction of thin film transistors and devices have been proposed wherein the various films of the transistor are made of different materials having different electrical characteristics. For example, thin film transistors have been proposed utilizing nickel oxide films, silicon films, amorphous silicon films and amorphous silicon and hydrogen films formed from silane as above mentioned. Also, various geometrical configurations have been proposed such as a planar-MOS construction.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine is introduced into the amorphous silicon semiconductor alloy to substantially reduce the density of localized states therein. Activated fluorine especially readily diffuses into and bonds to the amorphous silicon in the amorphous body to substantially decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine also combines in a preferable manner with silicon and hydrogen, utilizing the hydrogen in a more desirable manner, since hydrogen has several bonding options. Without fluorine, hydrogen may not bond in a desirable manner in the material, causing extra defect states in the band gap as well as in the material itself. Therefore, fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its high reactivity, specificity in chemical bonding, and high electronegativity.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the alloy so formed has a lower density of defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states.
Unfortunately, as reported by Le Comber and Spear and others, the silane based transistor devices had switching frequency speeds of about only 10.sup.4 Hz. As a result, while transistor devices made from amorphous silicon alloys are desirable from the standpoint of ease of manufacture, small size, and the ability to be made over large areas, their switching speeds have not been fast enough for high speed applications such as computer logic applications.
In view of the extreme commercial value and importance of solid state devices made from amorphous semiconductor alloys, considerable work has been done to improve the switching speeds of these devices. Matsumura, for example, as reported in Japanese Journal of Applied Physics, Vol. 20, No. 6, June, 1981 at pp. L414-L416, has theorized that under dynamic conditions the mobility of these devices becomes high and that relatively high speed switching is possible. However, in that publication, Matsumura merely theorized the existence of this phenomenon and did not show or suggest any way to take advantage of this theorized characteristic in a device or circuit.